In absorption refrigeration units it is common practice to cool the hot strong absorbent solution leaving the vapor generator by providing heat exchange between this solution and the relatively cold weak absorbent solution (rich in refrigerant) which is about to enter the generator. The heat transfer coefficient of a slow-moving non-boiling liquid is low compared to that of a boiling liquid, and therefore in this kind of arrangement the temperature of the weak absorbent solution leaving the heat exchanger is well below the boiling temperature. The heat input to the generator and hence the heat transfer area therein must therefore be sufficient to overcome this low coefficient in raising the solution to its boiling point. When a high temperature source such as a gas flame is available the resulting high temperature differential in the generator readily produces the required heat transfer.
If a relatively low-temperature heat source, such as a heat exchange liquid heated in a solar energy collector, is used in an absorption refrigeration system it can result in an appreciable increase in the heat transfer area required in the generator. This results in the disadvantages of increased complexity of construction and increased generator size and cost.
According to the principles of the present invention the solution which is rich in refrigerant just prior to entering the vapor generator, is passed in heat exchange relationship with the stream of heating medium leaving the generator, in a manner to preheat the temperature of the rich solution to a temperature very close to its boiling point. This can be accomplished by providing a solution pump to increase the normally low velocity of the rich solution and thereby increasing the coefficient of heat transfer in the preheater. Preferably the flow of solution along the heat exchange surface in the preheater is turbulent flow.